It's a possibility, but not a probability.
The big picture is that historically Mac OS has been "the choice" while Windows has been the defacto standard that everyone has been buying (because "everything" has been developed for Windows and fewer software titles for Mac). Now what Steve Jobs is trying to do is reverse the situation: Imagine Mac OSX being "the platform" where you can run and compile for "everything". That makes using Windows a choice of not having all options open! That makes more (technology conscious) people switch to OSX and more importantly; that makes more developers switch to OSX and/or XCode.
I think the first chap had it right. Being able to 'compile for everything' is not something that interests 96% of Apple's consumer base.
A future 'OSX' based on Windows would be able to run Windows applications and games at native speed. There would be no need to try and get games developers to write for the Mac, because they wouldn't have to. The likes of Adobe and Microsoft certainly won't be porting their code to 64-bit Cocoa because it wouldn't be worth the effort; what if they already know that they won't have to port anything?
It could be that this move is part of a transition; to make sure that the Cocoa developers are not abandoned if/when Apple decides to run their environment on top of Windows.
You can already see signs of this happening as game houses are coming back to Mac. Once these compile-for-all-platforms development tools are ready and games are developed with XCode (which is likely to happen, because if something is easier for a company like in developing software once instead of twice, that will happen), it makes Windows a 2nd grade platform. But there's no chance for it unless XCode can also compile for Windows. Until that happens, Mac games will always be ported.
Or to look at it realistically; these companies could still not be convinced to port to a Mac, so they are going to use an emulation layer, the performance of which (especially on the sub-par graphics of Apple's consumer machines) is already in question. It doesn't make Windows a 2nd grade platform, it makes the Win32 API the standard for everyone. Not the result many would like to see.